1. Field of the Disclosure
The present application is generally directed to radiation detectors comprising scintillators.
2. Description of the Related Art
Radiation detectors incorporating scintillators, commonly in the form of single crystal materials, are used in applications ranging from oil well logging to medical imaging. Typically, such detectors include one or more scintillators optically coupled to one or more photodetectors able to convert light into electricity. When such a detector is subjected to a radiation event, the scintillator generates light in response to the radiation, and the photodetectors may convert the light into electricity, which is used to record the event electronically.
Some radiation detectors, such as in medical imaging, are capable of imaging based on a plurality of radiation events. Medical imaging is usually accomplished by introducing into a patient a radiopharmaceutical substance by injection, ingestion, inhalation, or other appropriate means. A radioactive isotope of the radiopharmaceutical selectively migrates to the tissue to be examined and emits gamma radiation from it. The radiation can be sensed and used to generate an image of the tissue to provide diagnostic information for appropriate treatment.
The sensing of radiation from a biological tissue is typically accomplished by means of a detector commonly referred to as a gamma camera. Such a camera may be used in PET (Positron Emission Tomography) or SPECT (Single Photon Emission Computed Tomography) modalities and may feature a detector head including a round or rectangular camera plate optically coupled to a corresponding two-dimensional array of position-sensitive photodetectors, typically photomultiplier tubes (PMTs). The array of photodetectors may have a view of the camera plate which is typically about 30 centimeters or more in its major dimension. Detector heads weighing hundreds of pounds are used to make two-dimensional images, sometimes in a stationary mode and sometimes in a scanning mode. They can also be used to make three dimensional images by taking a plurality of views of the same target from different angles and using computer logic image reconstruction techniques. This may or may not include the use of time of flight measurements.
A so-called “gamma camera plate” is a large area device for converting radiation to light and is most commonly an assembly of a scintillator in the form of a crystal slab, such as sodium iodide doped with thallium for activation, which is hermetically sealed in a housing. The housing is made up of a shallow aluminum pan “back cap” covered with a glass optical window bonded to the back cap about its perimeter. An optical interface is typically provided between the crystal and the window to improve the coupling.
In operation, radiation from the target enters the crystal from the back cap radiation entrance side of the camera plate. The radiation interacts with the scintillator to result in scintillation light inside it. The light passes out of the plate through the optical window and into an array of PMTs which are coupled to its outside surface to convert the light to electrical signals. The electrical signals are fed to a digital processor for the construction of image information in a graphic form. The processor software may have the capability for accounting to some extent for spreading of the light inside the crystal between the point of its creation and its exit from the window into the photomultipliers. The spreading results in some loss of reconstructed image resolution and is undesirable in that respect, but it is at the same time also necessary to some extent for determination of position information by comparing the signal response of several nearby photomultiplier tubes to the same scintillation event.
In imaging applications, radiation detectors continue to suffer from lower resolution at the edges of the detector (“edge effect”), which may reduce the useful area of the scintillator. This effect is caused by radiation events occurring near an edge of the scintillator that are reflected from that edge making it more difficult to accurately determine the position of such events. In practice, it is not always possible to center the detector over the area of interest, making edge resolution potentially important. Various solutions to edge effect have been proposed, including the use of diffuse reflectors (see, for example, U.S. Pat. No. 4,284,891 to Pergrale et al.) and one or more light guides provided in a peripheral region of a scintillator or optical window close to an edge (see, for example, U.S. Pat. No. 7,138,638 to Juni).